Method and device for separating hydrocarbon fluids

ABSTRACT

A method for handling of hydrocarbon gas which is conducted to a flare for burning off is described. The method is characterised in that the hydrocarbon gas is separated into two or more fractions (parts), of which one fraction which has the desired quality is led to be burnt offin the flare, while the other fraction(s) is recovered. A device for carrying out the method is also described, and an application thereof.

The present invention relates to a method and a device for handling ofhydrocarbon fluids (gases) that are meant to be led to a flare to beburnt off in connection with oil, gas and product installations, asdescribed in the introduction in the subsequent claims 1 and 7,respectively.

The invention also relates to a particularly preferred application ofthe invention.

The aim of the invention is to reduce flaring and at the same time get acleaner flame without smoke.

It is often a requirement that flares installed on oil and gas andproduct installations shall not have visible smoke. Smoke is due toincomplete combustion, and often occurs when heavier hydrocarbons areburnt off. At the same time, these hydrocarbons are as a rule the mostvaluable, and they can be sold on the market as clean products, ascondensate or as a part of an oil product which is exported.

Hydrocarbons consist of a mixture of combustible molecules, i.e.everything from methane, ethane, propane and molecules with a highermolecular weight (i.e. with a higher number of carbon atoms in themolecule chain). The higher the average molecular weight (largerfraction of heavy components) a mixture has, the stronger the tendencyfor incomplete combustion and formation of soot particles. The “lighter”the mixture is (mainly such as ethane and methane), based on thementioned molecular weight, the cleaner the flame and the less soot onegets from the combustion. A hydrocarbon mixture that contains a largerfraction of hydrocarbons, from propane and butane upwards, is regardedas a so-called “heavy” mixture.

A possible way to take care of such heavy components in the flare gas isto close the flare with respect to the methods that are described in API521 (API—American Petroleum Institute).

One of these methods is to close the flare by means of a liquid seal.According to another of these methods, the flare is closed with the aidof valves and safety equipment in parallel. This method is described inNorwegian Patent Application 1993 1596 (NO Patent 177.161 (Statoil)) andimplies that the surplus gasses are recovered. It is normally inemergency situations that the surplus gases are led to the flare to beburnt off.

The present invention relates to a further development in the form of anadditional installation to the methods that are described in API 521 andas described in the abovementioned Norwegian patent.

It is an aim of the invention to provide a new method that can eliminatethe disadvantages that exist in today's handling of gases which shall beflared.

Thus, it is an aim of the invention to remove the gas components whichgive the most polluting soot-containing, and thereby, black smokedischarges. These components are made up of the heavy hydrocarbons witha higher average molecular weight which result in that they do not burncompletely or remain unburned, and thereby form soot particles and theassociated pollution.

Furthermore it is an aim to be able to avoid the large investments, theextra flaring, and necessary shutdown of the site which otherwise willbe necessary to avoid this soot-containing smoke.

It is a further aim of the invention to provide a method by which onecan in a simple way retain the valuable components that would be able togive an economic contribution (an income) to the process.

It is yet another aim of the invention to provide a method by which onein a simple way can reduce the discharges of CO₂ and unburnedcomponents.

The method according to the invention is characterised by the featuresthat are evident in the characteristics in the subsequent claim 1 andthe associated dependent claims.

The device according to the invention is characterised by the featuresthat are evident in the characteristic in the subsequent claim 7 and theassociated dependent claims.

Further features of the inventive device are given in the dependentclaims.

According to the invention the method and device to remove the “heavy”hydrocarbon components which form the soot when burnt from the “light”hydrocarbons that shall be burnt off in a flare, is applied as itappears in claim 13. The “heavy” hydrocarbon components that form thesoot when burnt are preferably returned to the source process which isprocessing of crude oil as described in claim 14.

According to the invention a system can be installed in parallel, inwhich a fraction of the flare gas is recovered. Such a system canincorporate a membrane pack that separates the heavier hydrocarbons fromthe light hydrocarbons. The heavier hydrocarbons are retained while thelight hydrocarbons are still burned. In this way one will obtain partialrecovery of flare gases, with the most valuable components beingrecovered while the other components are flared.

With the invention one has thereby achieved that a larger fraction ofthe hydrocarbon gas (with a larger fraction of heavy components in themixture) can be sent to recovery, while the remaining gas, which in themain consists of light hydrocarbons, can be burned off in the flaretower with a clean flame without smoke, or that the light components aredischarged unburned in the flare (cold flare).

The separation element can be a membrane that separates the heavyhydrocarbons from the light hydrocarbons. A membrane normally needspressure support from the feeding side and/or the aid of suction on thepermeate side. The separation element will therefore often in additionto membranes encompass compressors upstream of the membranes or vacuumpumps downstream of the membranes.

Thus, the membrane functions by separating the original mixture into twofractions. One fraction gets a reduced overall molecular weight andburns with a cleaner flame, while the other fraction gets an increasingproportion of molecules with a higher molecular weight, and is therebywanted returned to the process.

An example of a concrete embodiment or solution can be that the flare isclosed, for example, by the aid of a liquid seal or a valve with safetyequipment, in the form of pipes with valves in parallel, and, forexample, comprising a safety valve (with rupture disc). In parallel tothis, the membrane is installed which separates out the heaviercomponents that are recovered, for example, with the aid of acompressor, and are led to a suitable location. The membrane is fittedin a chamber/housing-forming separation unit. Because of theback-pressure which the liquid seal (or safety equipment with valves)exerts against the fluid stream in the main pipe, the gas is led throughthe pipe to the chamber with the membrane. If too much gas flows throughthe pipe 10 so that the pressure increases, the gas will gradually beginto flow through the liquid seal (valve system) and thereby relieve thepressure increase.

This solution will be interesting for many installations. In particularinstallations that have no export possibilities for gas, but can handlecomponents in liquid form. These installation will be able to retain theheavier components and lead them to recovery (for example back to thecrude oil), and will with the aid of this solution obtain possibilitiesto increase the income as well as reduce polluting discharges.

Installations which have problems with smoke discharges will also beable to reduce, possibly completely eliminate, this problem byimplementing this solution.

In the following the invention shall be explained further with referenceto the enclosed figures, in which:

FIG. 1 shows an installation which handles a gas, such as surplus gases,for separation, according to the invention.

FIG. 2 shows the same separation system as in FIG. 1, but comprises analternative solution for the safety system.

A flare gas comprised of hydrocarbons is conducted through a pipe 10from a source, not shown, and to a liquid seal 12 that comprises achamber 14 filled with liquid. The end 13 of the pipe 10 extends downinto the liquid. A flare pipe 16 (a tube) protrudes from the upper partof the liquid seal 14 and leads the gas on to the flare 18 itself. Thepressure which the gas in the pipe must have to rise further upwards inthe flare tube, depends on how far the end 13 extends down into theliquid below the surface 15. Thus, the liquid seal construction providessafety if pressure increases arise in the tube 10. The gas will then beled out in the flare 18.

As a consequence of the pressure which the liquid seal in the flare tube16 sets up, the gas is led through the pipe 22 to achamber/housing-forming separation unit 24 that comprises a membrane 26.A membrane 26 is applied which will separate the gas into twofractions/flows of heavy and light hydrocarbon components, respectively.The light components are led through the pipe 28 and directly into theflare tube 16 to be burnt off in the flare 18. If too much gas comesthrough the pipe 10, so that the pressure increases, the gas willgradually begin to flow through the liquid seal (or the valve system)and thereby relieve the pressure increase.

From the other side of the membrane 26, a gas fraction of heavycomponents (i.e. the gas fraction that passes through the membrane) isled further through the pipe 30 to a compressor 32 where it iscompressed and led into a process 34 for further treatment.

The further treatment can involve that the compressed fraction, togetherwith crude oil, is conducted to further processing.

As the gas fraction of the light components which shall be burned off inthe flare 18 now contains light components only, one obtains a cleanflame without soot-forming, polluting, partly burnt hydrocarbons.

As an extra safety arrangement (or as a replacement for the liquid sealconstruction), a bypass pipe which can cut off the supply of gas to theseparation unit 24 can be installed. The bypass pipe is initially closedwith the aid of parallel-coupled safety valves, including an ordinaryvalve and a rupture-disc valve. The valve(s) in the pipe can be openedto lead gas directly to the flare 18 if faults arise in the separationunit 24, or unwanted pressure build-ups occur in the system, or otherfaults arise. The solution for the safety system with parallel pipes40,42 between the inlet pipe 10 and flare tube 16, and with theinstalled valves 44,46 are outlined in FIG. 2. The valves 44,46 are setso that they open for flow to the flare tube 16 when the pressure is toohigh. One of these valves can include a safety-valve with a rupture discwhich ruptures at a given higher pressure.

The separating component 26 is, as mentioned, a membrane which separatesout the heavy hydrocarbons from the light hydrocarbons. The membrane 14needs pressure-support on the feeding side, or the help of suction onthe permeate side. Therefore, in addition to the membrane, one (or more)compressor(s) upstream of the membrane and/or vacuum pumps downstream ofthe membranes, are often installed. The membrane dissolves the heaviercomponents and releases them on the other side of the membrane.

With the present invention, a solution is provided which will eliminatethe problem of discharging solid soot particles from flares, somethingwhich constitutes a real pollution problem in addition to it beingaesthetically unattractive.

The abovementioned example shall only be considered as an example, asthe invention in its widest context is as described in the subsequentclaims.

The mentioned solution will be interesting for many installations. Inparticular, installations which do not have possibilities for exportinggas but which can retain components in liquid form. These installationswill get possibilities, by the aid of this solution, to increase theincome as well as reduce discharges.

Installations which have problems with smoke discharges will also beable to reduce, possibly eliminate, this problem with the solutionaccording to the invention.

1. Method for handling of hydrocarbon fluid (gases) which are led to a flare to be burnt off, characterised in that the hydrocarbon gas is separated into two or more fractions (parts), whereby one fraction which has the desired quality is led to be burnt off in the flare, while the other fraction(s) is recovered.
 2. Method according to claim 1, characterised in that the hydrocarbon gas is separated by it being led through a membrane which separates light (hydrocarbon) components from heavy components, as the light components are led to the mentioned burning off.
 3. Method according to claims 1-2, characterised in that the heavy hydrocarbon components are compressed and condensed to a liquid state and are mixed with a main product from the process which forms the combustible gases, said main product can be crude oil.
 4. Method according to claims 1-3, characterised in that a safety system is applied in the form of a bypass of the fluid flow directly to the flare tube (16).
 5. Method according to claims 1-4, characterised in that a pipe system with a liquid seal is used in the bypass.
 6. Method according to claims 1-4, characterised in that for the bypass parallel pipes between the inlet pipe and flare tube are used, and with respective valves inserted, said valves open for flow to the flare tube at too high pressure, as a safety valve (with rupture disc) is used as one of the valves.
 7. Device for handling of combustible hydrocarbon fluids (gases) which are intended to be burnt off in a flare, characterised by means for separation of gases into two or more fractions (parts), means for leading a fraction which has the desired quality to the combustion, and means for transfer of the other fractions to recovery.
 8. Device according to claim 7, characterised in that the separation means comprises a membrane which can separate light (hydrocarbon) components from heavy components.
 9. Device according to claims 7-8, characterised in that a safety system comprising a bypass of the fluid stream directly to the flare tube.
 10. Device according to claims 7-9, characterised in that the bypass comprises a pipe system with a liquid seal.
 11. Device according to claims 7-10, characterised in that the bypass comprises parallel pipes between the inlet pipe and flare tube, and with inserted respective valves, said valves open for fluid flow to the flare tube when the pressure is too high, as a safety valve (with rupture disc) is used as one of the valves.
 12. Device according to claims 7-11, characterised by compression equipment upstream or downstream of the membrane to obtain desired fractions of the hydrocarbons.
 13. Application of method and device according to the preceding claims for the removal of the “heavy” hydrocarbon components that form soot when being burnt from the “light” hydrocarbons that shall be burnt off in a flare.
 14. Application according to claim 7 for return of the “heavy” hydrocarbon components that form soot when being burnt to the source process which is processing of crude oil. 